A magnet pump including front and rear casings composed of synthetic resin has been employed for the use of transferring a corrosive liquid, for example. In such the magnet pump, the front casing forms a pumping chamber and the rear casing forms a cylindrical space contiguous to the pumping chamber. The cylindrical space in the rear casing is employed to locate a cylindrical magnet can that is rotatably supported by a support shaft, one end of which is secured on the rear casing. Outside the magnet can, a rotary driver is located and magnetically coupled to the magnet can through the rear casing. The driving force from the rotary driver is employed to rotate the magnet can. The magnet can is integrally coupled to an impeller, which is housed in the pumping chamber. When the impeller rotates, a target fluid to be transferred is introduced into the inside of the pumping chamber through an inlet formed at the front of the front casing and discharged through an outlet formed at the side of the front casing.
The rotator consisting of the magnet can and the impeller has a sliding portion, which is located at the inner diameter side in the vicinity of the impeller inlet. Therefore, if bubbles are mixed into the target fluid, the bubbles concentrate inwardly due to a difference in their specific gravity and invite an incomplete cooling action by the target fluid at the sliding portion, resulting in the sliding portion heated easily. A spindle boss arranged in the vicinity of the sliding portion has just a small clearance with peripheral members and hardly radiates heat. Thus, the conventional magnet pump with the synthetic resin casing causes a problem because the synthetic resin casing may be deformed or melted due to the heat caused by mixed bubbles and poor heat radiation.